In technologies for transmitting voice data, there has been used a technique that encodes voices by means of a vocoder to transmit them such as, for example, that described in a prior document, “PERSONAL DIGITAL CELLULAR TELECOMMUNICATION SYSTEM RCR STD-27 Revision J,” by Association of Radio Industries and Businesses, May 30, 2002. Recently, with the development of encoding technologies, sufficiently natural voices can be transmitted in real time even at a much low bit rate on the order of 2000 [bps] or less, using a small amount of code: this technique has also been used in such applications that must use a transmission channel whose communication quality is not necessarily good, such as an automobile telephone system.
When voice data is transmitted at a low bit rate, a small amount of bit errors may have serious effects on the quality, and therefore, it is essential to accurately detect or correct the errors.
In doing so, there has been used a technique, for example, in which a device that transmits voice data adds cycle redundancy check (CRC) codes to the coded voices, and a receiving device uses the CRC codes to correct errors.
However, in case of voice data that is small in the amount of the data, and when the number of bits of the CRC codes is given so as to ensure sufficient sound quality while the communication quality of the transmission channel remains within a normal range, the redundancy of the voice data will be excessively increased, and it will be difficult to achieve real time voice transmission.
As a technique to solve the problem, it is conceivable that communication quality of the transmission channel is to be judged and only transmitted voice data in a good condition in terms of the communication quality is to be used (specifically, for example, voices to be reproduced from transmitted voice data in poor communication quality are muted, etc.).
As a technique to judge communication quality, it is conceivable that, for example, when a sequence of symbols representative of coded voices are transmitted in the frequency shirt keying (FSK) modulated form, a receiving device measures an instantaneous value at a Nyquist point (where instantaneous values of the baseband signal are converged on any of multiple predetermined ideal values representative of symbols (the ideal value is also referred to as symbol value)) of the baseband signal obtained by demodulating the received FSK modulated wave, and judges the communication quality based on the difference between the measured value and the ideal value.
However, sufficiently fine level of sampling of the baseband signal and subsequent complex calculation are required to determine the difference between the measured instantaneous value of the baseband signal at a Nyquist point and the ideal value. This results in a complex construction of the device that receives the voice data, and it will be difficult to achieve real time voice transmission.
As another technique, it is also conceivable that, for example, when voice data has been subjected to the forward error correction (FEC), communication quality is to be judged on the receiving side of the voice data based on the number of error corrections identified in the process of error correction.
However, the process of providing the FEC, and the process of correcting errors in the voice data that has been subjected to the FEC are both complex. This results in a complex construction of the devices that transmit and receive the voice data, and it will be difficult to achieve real time voice transmission. In addition, the number of error corrections has a predetermined upper limit, and if the number of erroneous bits exceeds the upper limit, the number of erroneous bits cannot accurately be known based on the number of error corrections. Therefore, accurate judgment of the communication quality cannot be achieved.
As yet another technique, it is also conceivable that, for example, when voice data is wirelessly transmitted, the electric field strength of the voice data is to be measured and communication quality is to be judged based on the measured result, on the receiving side of the voice data.
However, if the voice data is mixed with noises, apparent electric field strength may increase and cause a high risk of errors in judgment of the communication quality. It is also conceivable that, for example, moving averages on a number of voice data are to be determined for use in measuring the electric field strength. In this case, however, it takes time to judge the communication quality, and multiple transmissions are required for the same voice data; it will be difficult to achieve real time voice transmission.
As yet another technique, it is also conceivable that, for example, communication quality is to be judged based on an ON or OFF state of squelch, assuming that the squelch circuit or the like is provided on the receiving side of voice data.
However, if the voice data is mixed with noises, the squelch may be incorrectly opened and in such a case, there is a high risk of errors in judgment of the communication quality. Additionally, in case where moving averages on a number of the voice data are determined in order to increase relative strength of the voice data to be received, it will be difficult to achieve real time voice transmission, as is the case with the above technique that uses a measured result of the electric field strength.